Conventional light guidance systems in an underground car park or public subways employ movement detectors in order to reduce power consumption by switching on light sources only, when a person passes by. In a situation of restricted power supply, however, switching on all light sources of the whole system will soon result in a supply shortfall. Therefore, in many of these systems, the intensity level of emitted light is lower than in a standard illumination for saving energy, although a higher light level would ameliorate the ability of orientation and the feeling of ease and comfort. Normally, these light guidance systems are powered by an external power supply system and the light sources are emitting at a low mandatory intensity level. For power cut situations, the light guidance systems and in particular emergency lighting systems are often provided with a backup power system so that the illumination can be kept up.
U.S. Pat. No. 7,355,349 B2 describes an emergency lighting system adjusting the emergency lighting level according to a detected luminance of surroundings and a detected motion of a person. However, if battery level is low and power is cut, a high-intensity illumination will lead to a decreased time period of illumination, possibly contradicting the norm standards.